In inkjet printing, printing is conducted by spraying a liquid ink with a high degree of fluidity from very fine nozzles, and adhering that ink to a recording medium such as a sheet of paper. This type of inkjet printing enables the printing of high-resolution, high-quality images at high speed and with minimal noise, using a comparatively inexpensive printing apparatus, and is rapidly becoming widespread. Inkjet printing is conducted mainly using thermal systems that use heat to spray the ink, piezo systems that use application of a vibration to spray the ink, and electrostatic systems.
In terms of the inks for these inkjet systems, aqueous type inks are becoming more prevalent, as they are capable of producing high-quality printed items at low cost. Aqueous inks are inks in which water is included to enhance the drying properties, and compared with oil-based inks, offer the advantages of resistance to show-through (penetration of the ink through to the underside of the recording medium) even if double-sided printing is conducted onto plain paper, and favorable text reproducibility. The colorants used for these aqueous inks can be broadly classified into dyes and pigments, and pigment inks are rapidly gaining in popularity, as they offer superior levels of light resistance and water resistance.
Examples of problems that may occur when printing an aqueous ink onto paper such as plain paper include the phenomena of curling (convex curling) and cockling (wrinkling). These phenomena occur because the moisture within the aqueous ink causes cleavage of the hydrogen bonds between fibers within the paper, causing the paper to swell.
In order to address this type of curling problem, Patent Document 1 proposes an aqueous inkjet ink comprising water, a water-soluble organic solvent and a pigment, wherein the amount of the portion of the water-soluble organic solvent that has an SP value of at least 16.5 but less than 24.6 represents at least 30% by mass of the entire ink. A water-soluble organic solvent having an SP value within this range exhibits favorable compatibility with water, and also exhibits a curling inhibiting effect. By including at least 30% by mass of this type of water-soluble organic solvent, curling and cockling can be prevented.
[Patent Document 1] Japanese Patent Laid-Open No. 2007-153975